1. Field of the Invention
The invention relates to an electric power steering system for a vehicle, which generates steering assist torque by driving a motor on the basis of a driver's steering operation.
2. Description of Related Art
Conventionally, an electric power steering system uses a torque sensor to detect a steering torque input to a steering wheel by a driver, and calculates a target steering assist torque on the basis of the detected steering torque. A driver's steering operation is assisted by controlling a current flowing through a motor such that the target steering assist torque is obtained.
A motor that requires a rotation angle sensor may be used as the motor of the electric power steering system. For example, when a brushless motor, such as a three-phase permanent magnet motor, is used, it is necessary to detect a motor electrical angle with the use of a rotation angle sensor and control the phases of three phase voltages.
When the rotation angle sensor fails, it is not possible to control the phases of the motor. Therefore, there is also known an electric power steering system including two rotation angle sensors as suggested in, for example, Japanese Patent Application Publication No. 2012-98231 (JP 2012-98231 A).
When two rotation angle sensors are provided, it is possible to determine there is a failure in any one of the rotation angle sensors when two detected signals of the rotation angle sensors deviate from each other as a result of a comparison between the two detected signals. However, it may not be possible to accurately detect a failure of one of the rotation angle sensors after a failure of the other one of the rotation angle sensors has been detected. This is because it is not possible to compare detected signals of the rotation angle sensors with each other.
For example, when the magnitude of a detected signal (for example, voltage signal) of any one of the rotation angle sensors falls outside an assumed range, it is possible to detect a failure of the any one of the rotation angle sensors solely. However, for such a failure that the magnitude of a detected signal is fixed to an intermediate value within the assumed range (a failure due to a short circuit of a signal line, or the like), it is not possible to determine whether the detected value is fixed because of a failure or the steering wheel is locked. Therefore, it is not possible to properly monitor the rotation angle sensor, in which no failure has been detected, for a failure. In consideration of the fact that it is difficult to accurately determine whether there is a failure in the rotation angle sensor in which no failure has been detected when a failure of one of the rotation angle sensors has been detected, it is necessary to stop steering assist control even when the rotation angle sensor in which no failure has been detected is actually normal.
Therefore, even when the accuracy of detecting a failure of one of rotation angle sensors is improved by providing the two rotation angle sensors, it may not be possible to effectively utilize the other rotation angle sensor, in which no failure has been detected, after a failure of the one of the rotation angle sensors has been detected.